1. Field of the Invention
This invention relates to an aqueous cathodic electrodeposition lacquer coating composition which contains (A) one or more amino-group-containing synthetic resin binders that can be made soluble or dispersible in water by protonization with acids, (B) crosslinking agents reacting therewith on baking and (C) soluble and/or insoluble transesterification and/or esterification catalysts for the thermal crosslinking reaction at temperatures above 100.degree. C. and which optionally contain pigments, corrosion inhibitors, anti-cratering agents, lacquer auxiliaries, catalysts and organic solvents. The invention also relates to the use of this coating composition for cathodic deposition from a lacquer bath onto electrically conductive surfaces.
2. Description of the Related Art
Cathodic electrodeposition methods are described generally in U.S. Pat. Nos. 3,947,339 and 4,009,133, and additionally by Ranny, M. W. "Epoxy Resins and Products--Recent Advances," Noyes Data Corp., 1977, pp. 47-91.
DE-A-27 07 405 (U.S. Pat. No. 4,238,594) and DE-A-27 07 482 (U.S. Pat. No. 4,320,220)describes self-crosslinking binders for cathodic electrodeposition lacquers which are amino-group-containing reaction products and/or polyaddition products containing terminal or lateral double bonds.
According to DE-B-27 32 902 (U.S. Pat. No. 4,176,099) , reaction products such as these may be used together with urethane compounds as binders for cathodic depositions. Disadvantages of binders such as these include their relatively high content of organic solvents emanating from the production process, the formation of surface defects, the inadequate elasticity of the coatings and adhesion problems with further coatings applied thereto.
EP-B-66 859 (U.S. Pat. No. 4,430,462) describes binder systems consisting of:
(A) mixtures or precondensates of synthetic resins containing terminal or lateral double bonds, primary and/or secondary hydroxyl groups and amino groups,
(B) polyesters containing 8-hydroxyester groups as transesterification and/or transamidation hardeners and
(C) transesterification catalysts.
One disadvantage of this system lies in the T formation of clevage products during the baking process.
In German patent application P 35 30 179.1-43, the polyesters containing 8-hydroxyester groups are replaced by special crosslinking agents which are Michael's addition products of monocarboxylic or dicarboxylic acid esters capable of Michael's addition and containing at least one double bond capable of Michael's addition and which, on average, contain either two reactive ester groups or one reactive ester group and one ethylenic double bond per molecule. Clevage products are again formed during the crosslinking reaction.
US-A-3 925 181 and US-A-3 975 251 describe secondary amino groups with compounds containing double bonds. (CED is an internationally used abbreviation for cathodic electro-deposition.) In this case, the crosslinking reaction is a Michael's addition of reactive amines to double bonds. The disadvantage lies in the inadequate stability of the CED baths because the Michael's addition can actually take place at room temperature. The instability of the CED baths can be reduced by complete neutralization of the primary and secondary amines. Obtaining a good crosslinking density presupposes a sufficiently large quantity of reactive amino groups and, hence, a correspondingly large quantity of neutralizing acid. This increases the amount of electricity required for deposition and the danger of disruption of film formation by gas bubbles at the cathode and reduces throwing power. In addition, the basicity of the coating leads to poorer anti-corrosion behavior, particularly after mechanical stressing, for example by loose chippings. The adhesion of subsequent coatings, for example in automobile production, is seriously affected by the basic primer coating.